Arrangement for measuring the root mean square of a power frequency voltage across a large dynamic range, especially for measuring electrical output

ABSTRACT

The root mean square of a power frequency voltage is measured across a large dynamic range, for example in order to measure electrical output, by converting the voltage to be measured into two or more partial voltage proportional thereto and measuring each partial voltage using an individual diode rectifier. The output voltages of said diode rectifiers are then converted into a digital value and each of these digital values is weighted with weighting factors, the ratio of said weighting factors being derived from the control factor of at least one of the diode rectifiers. These weighted digital values are then scaled to a common quantification unit and added to achieved the actual digital measurement value.

The invention relates to, and is based on, an arrangement according tothe precharacterising clause of the main claim.

Arrangements of this type are known (U.S. Pat. Nos. 4,873,484 and4,943,764). The large dynamic range is divided up between two or morediode rectifiers, the output voltage of each individual diode rectifierbeing evaluated only when it is operating in a usable section of itsquadratic characteristic-response range. The output voltages of thediode rectifiers are fed via a measurement-range changeover switch to asingle evaluation circuit, i.e. the output voltage of only a singlediode rectifier is ever evaluated. If, in the event of abrupt changes inthe measurement quantity or owing to modulation, the usablecharacteristic-response section of the currently active diode rectifieris departed from, this makes it necessary to terminate the measurementtaking place, switch over to a favourable diode rectifier, i.e. onewhich is operating in the usable characteristic-response range, wait forthe transient settling time of the evaluation circuit to elapse, andstart a new measurement. This makes it difficult or impossible, inparticular, to observe one-off (transient) processes which are notcovered by the dynamic range of a single diode rectifier. Especially inthe case of radiofrequency signals with a time-varying envelope-curvepower, in which the ratio between the maximum value and the averagevalue of the envelope-curve power is very large, such known methodswhich operate with range changeover have the disadvantage that themeasurement range must be selected in such a way that individual briefpower peaks do not overdrive the circuit. The effect of this, however,is that the circuit is driven at a low level for most of the time, sothat the signal-to-noise ratio is small.

It is an object of the invention to provide such an arrangement formeasuring the rms value of a voltage over a large dynamic range, inwhich these disadvantages are avoided and which permits accurateerror-free measurement even in the event of a time-varyingenvelope-curve power, for example in response to abrupt changes in themeasurement quantity or owing to modulation.

This object is achieved, on the basis of an arrangement according to theprecharacterising clause of the main claim, by the characterisingfeatures thereof. Advantageous refinements are given in the dependentclaims.

In the arrangement according to the invention, a changeover switch isobviated and, for each diode rectifier, a separate channel is providedto treat the voltage components measured by the diode rectifier. In thiscase, the term “voltage component” is intended to mean either a fractionor a multiple of the input voltage to be measured. Separateanalogue/digital conversion with subsequent digital signal processingtakes place in each channel. When very fast A/D converters are used, thedigitising of the separate channels could also take place sequentially,i.e. in these cases it is sufficient to have only a single A/Dconverter, the processing path being split again into two separatebranches immediately after this A/D converter. The digital values of allthe channels are correspondingly weighted according to their informationcontent and are scaled. The sum of the weighting factors is in this caseone. The individual weighting factors, or their ratio, is derived fromthe drive level of at least one of the diode rectifiers. The measurementchannel for the higher powers will, for example, deliver measurementresults at the lower limit of its measurement range which have greatermeasurement errors than in the more sensitive measurement channel. Withincreasing power, the situation will be reversed, for example owing tooverdrive effects in the more sensitive measurement channel. Dependingon the relationship between measurement error and power, differentweighting functions will therefore be used in the context of thedependent claims. So that the results from the individual channels cansubsequently be added to give an output value, the weighted digitalvalues of the individual channels furthermore need to be scaled in sucha way that they can be represented as a fraction or as a multiple of acommon quantisation unit.

An arrangement according to the invention is suitable not only for theaccurate measurement of radiofrequency signals with a constantenvelope-curve power as a function of time (CW signals) but also, aboveall, for the accurate measurement of radiofrequency signals with atime-varying envelope-curve power, as is the case for example withmodulated radiofrequency signals. This is because, in the arrangementaccording to the invention, a measurement value is obtained, for eachindividual sample value of the A/D converter, by weighting and scalingthe corresponding digital value.

An arrangement according to the invention for measuring the rms value ofan AC voltage over a large dynamic range is suitable not only formeasuring the electrical power of, for example, radiofrequency signals,but also for measuring the rms value, or root mean square value, of anyother physical quantities over a large dynamic range, which areconverted into a voltage by appropriate transducers, as is the case forexample when measuring mechanical stresses by a measurement transduceror the like. The arrangement according to the invention can also be usedin the same way in this case.

The invention will be explained in more detail below with the aid ofschematic drawings with reference to an exemplary embodiment.

FIG. 1 shows the block diagram of a circuit according to the inventionfor measuring the electrical power over a large dynamic range.

First, in order to divide up the dynamic range, the radiofrequencysignal with the power p_(i) to be determined is fed to the linearnetwork A, which uses suitably arranged splitters, attenuators oramplifiers to generate two radiofrequency signals with different powersp₁ and p₂ proportional to p_(i). Let the powers p₁ and p₂ bedimensioned, in relation to one another, so that the usablecharacteristic-response ranges of the two diode rectifiers B1 and B2partially overlap, i.e. there is an overlap region in which both B1 andB2 deliver valid results. Let the relationship p₁>>p₂ always besatisfied in this case, i.e. let channel 1 be substantially moresensitive for p₁ than channel 2 is, with the structure of both channelsbeing identical.

The two generated radiofrequency signals are rectified by using thediode rectifiers B1 and B2. Since B1 and B2 are operated in thequadratic range, their output voltages ν₁ and ν₂ are respectivelyproportional to p₁ and p₂. With the aid of the choppers C1 and C2, theDC voltages ν₁ and ν₂ are converted into the AC voltages ν₁′ and ν₂′, inorder to prevent the measurement result from being compromised by theoffset-voltage drifts of the amplifiers D1 and D2 and the zero-pointdrifts of the analogue/digital converters E1 and E2. This procedure canbe regarded as multiplication by a variable c which alternately takesthe values +1 and −1.

The AC voltages ν₁′ and ν₂′ are amplified by the amplifiers D1 and D2,respectively. The amplified AC voltages ν₁″ and ν₂″ are subsequentlyconverted by the analogue/digital converters E1 and E2 into the digitalvalues x₁ and x₂, respectively. Analogue/digital conversions in E1 andE2 are initiated simultaneously by the common control signal CONV.

The digital values x₁ and x₂ are affected by offsets, i.e. in spite of alinear relationship with p₁ and p₂, respectively, they are notproportional to p₁ and p₂. Only after an offset correction, during whichthe offsets o₁ and o₂, respectively, are subtracted from x₁ and x₂ withthe aid of the adders F1 and F2, are digital values x₁′ and x₂′ oncemore obtained which are proportional to p₁ and p₂, respectively. Theoffsets may be different for the two switching states of the choppers(e.g. owing to thermal emfs which have their origin in the dioderectifiers). Before the measurement, they are determined and stored forboth switching states of the choppers (c=+1 and c−1) by measuring x₁ andx₂ for p_(i)=0, and are stored.

The function blocks G1 and G2 are used, if necessary, to linearise thecircuit's characteristic response. In the present application, there isalready a linear dependency of the output voltage of the individualdiode rectifiers on the power supplied to them. Since all the othercircuit components likewise exhibit a linear behaviour, G1 and G2 may beomitted in the ideal case. Often, however, it is desirable to drive thediode rectifiers B1 and B2 beyond the upper limit of the quadraticrange. The resulting linearity discrepancies can be corrected for by G1and G2, in order to obtain a larger usable characteristic-responserange. It should nevertheless be borne in mind that, when the dioderectifiers are driven beyond the upper limit of the quadratic range,harmonics contained in the radiofrequency signal cause highermeasurement errors.

If it is not the power but, instead, the voltage rms value of aradiofrequency signal which is to be determined by the describedcircuit, then G1 and G2 can be used to compensate for the quadraticdependency of the output voltage of the individual diode rectifiers ontheir input voltage, i.e. in this case they are used to calculate thefunction x″₁={square root over (x′₁)} or x″₂={square root over (x′₂)},respectively. Further specific applications for G1 and G2 are obtainedwhen using other measurement transducers with a non-linearcharacteristic response, e.g. in order to measure physical quantitiesother than the power or the voltage rms value of a radiofrequencysignal.

After the linearisation, the digital values x₁″ and x₂″ are weightedaccording to their information content, through use of the multipliersH1 and H2 to multiply them by the weighting factors w₁ and w₂. Theseweighting factors are generated in the function blocks H1 and H2. Thedegree to which the analogue/digital converter E1 is driven, forexample, can be used as a measure of the information content:

If E1 is e.g. positively or negatively driven to less than one half,then E2 is driven much less (channel 2 was assumed to be substantiallyless sensitive than channel 1). The results of channel 2 then have apoor signal-to-noise ratio, so that the information content is to beregarded as low. In this case, w₁=1 and w₂=0 (FIG. 2). The results ofchannel 2 are discarded, and the results of channel 1 are furtherprocessed to 100%.

If E1 is e.g. positively or negatively driven to at least one half, butnot yet fully, then both channel 1 and channel 2 give valid results. Theweighting of channel 1 decreases gradually as the driving of E1increases. Conversely, the weighting of channel 2 increases gradually.The sum of all the weighting factors must always be equal to 1.

If E1 is positively or negatively driven maximally, the results ofchannel 1 have a low information content because, with further driving(overdrive), the output digital value can no longer become greater(clipping). Consequently, w₁=0 and w₂=1. It is technically expedient totune the full driving of the analogue/digital converter in each channelto the upper limit of the usable characteristic-response range of thediode rectifier.

The weighted digital values x₁′″ and x₂′″ are multiples of differentquantisation units, i.e. the same digital values represent a differentpower in channel 2 than in channel 1. This is the effect, on the onehand, of the network A and, on the other hand, of the different gain ofD1 and D2 and the different sensitivity of B1, B2, E1 and E2 (even withfully identical dimensioning, tolerances can never be completelyavoided). The multiplication by the scaling factors s₁, s₂, with the aidof the multipliers J1 and J2, compensates for these effects. The scalingfactors s₁, s₂ are e.g. dependent on the carrier frequency of theradiofrequency signal. They are determined by a calibration procedureand are stored. The digital values x₁″″ and x₂″″ are multiples orfractions of the same quantisation units, i.e. the same digital valuesrepresent the same powers. They are added with the aid of the adder K.

The synchronous demodulator L makes it possible to reverse the action ofthe choppers C1 and C2. This is done through multiplication of y by thevariable c. The digital value y already contains the variable c owing tothe effect of the choppers. The rectified digital value z is ideallyindependent of c, because it contains this factor twice, which leads tocompensation for it (1·1=1 and −1·−1=1). In practice, however, theaforementioned offset correction by using F1 and F2 is never fullyachieved: The actual offsets can change constantly e.g. owing totemperature fluctuations. The adders F1 and F2, however, subtract onlythe offsets which were determined and stored at a particular time.Moreover, all the offset contributions which are created in the circuitdownstream of C1 and C2 are multiplied by c only once, i.e. they areconverted into AC voltages by L. Low-frequency noises from D1, D2, E1and E2, whose frequency lies below the chopper frequency, are mixed withthe chopper frequency by the synchronous demodulation, and are therebyspectrally modified in such a way as to facilitate, or actually makepossible, subsequent noise suppression through digital filtering.

The obtained digital value z is proportional, over a large dynamicrange, to the radiofrequency signal power p_(i) to be measured. It canbe processed further in a conventional way. To that end, use is made ofthe circuit block M, which contains e.g. the digital filtering of themeasurement values, the compensation for the temperature excursion, thecalculation of the voltage rms value, control functions and themeasurement-value display.

As already mentioned, the characteristic response of a circuit accordingto the invention does not have any discontinuities and hysteresiseffects. This ensures that a particular power p_(i)=p₀ always has aunique displayed power value p_(d) allocated to it (FIG. 3).

The precise function of the chopper and the synchronous demodulator canbe derived from FIG. 4, which represents an example of the time profileof the variable c. The measurement task involves determining the averageburst power of the signal depicted in the upper diagram. To that end,under the control of a trigger circuit, measurement data is recorded andaveraged only within the measurement intervals of duration T_(a)(aperture time). One power-average value P_(v) is obtained permeasurement interval. The sign of c is changed at the end of eachmeasurement interval. The time until the start of a new measurementinterval must be large enough for the transient settling processes inthe circuit to be able to decay. An even number of power-average valuesP_(v) is always recorded. This ensures that any interfering offset,which is created in the circuit downstream of the choppers and is notfully corrected by F1 or F2, occurs with a positive sign in exactly asmany P_(v) as it does with a negative sign. If the arithmetic mean ofall the P_(v) is taken, then this offset is compensated for fully. Theactual measurement value P is obtained as a result of the averaging.

In the aforementioned example, the time T_(burst) which indicates theperiod of the signal burst, lies between two sign changes of c. Thechopper frequency is therefore 1/(2 T_(burst)). If the aperture time andthe chopper frequency are not dictated by the measurement task, then acircuit according to the invention automatically selects expedientstandard values for them.

Of course, the type and arrangement of the function blocks may vary in acircuit such as the one represented in FIG. 1. Owing to their linearnature, for example, the order of the multipliers may be swapped or thesynchronous demodulation may be shifted to before the summation.Likewise, the represented separation into analogue and digital functionblocks is merely to be regarded as being technically expedient, andwithout implying any limitation. For instance, it is conceivable torelocate the offset correction into the analogue part of the circuit.

What is claimed is:
 1. Arrangement for measuring the rms value of an ACvoltage over a large dynamic range, in particular for measuringelectrical power, in which the voltage to be measured is converted intotwo or more voltage components proportional thereto, and each voltagecomponent is measured by using a separate diode rectifier (B1, B2),characterised in that the output voltages of the diode rectifiers (B1,B2) are converted into a digital value, each of these digital values isweighted with weighting factors (w1, w2) whose ratio is derived from athe control drive level of at least one of the diode rectifiers (B1,B2),and, after scaling to a common quantisation unit, these weighted digitalvalues are added to give the actual digital measurement value. 2.Arrangement for measuring the rms value of an AC voltage over a largedynamic range, in particular for measuring electrical power, in whichthe voltage to be measured is converted into two or more voltagecomponents proportional thereto, and each voltage component is measuredby using a separate diode rectifier (B1, B2), characterised in that theoutput voltages of the diode rectifiers (B1, B2) are converted into adigital value, each of these digital values is weighted with weightingfactors (w1, w2) whose ratio is derived from a control drive level of atleast one of the diode rectifiers (B1,B2), and, after scaling to acommon quantisation unit, these weighted digital values are added togive the actual digital measurement value, and characterised in that thesum of the weighting factors is one.
 3. Arrangement for measuring therms value of an AC voltage over a large dynamic range, in particular formeasuring electrical power, in which the voltage to be measured isconverted into two or more voltage components proportional thereto, andeach voltage component is measured by using a separate diode rectifier(B1, B2), characterised in that the output voltages of the dioderectifiers (B1, B2) are converted into a digital value, each of thesedigital values is weighted with weighting factors (w1, w2) whose ratiois derived from a control drive level of at least one of the dioderectifiers (B1,B2), and, after scaling to a common quantisation unit,these weighted digital values are added to give the actual digitalmeasurement value, and characterised in that the output voltages aredigitised by using a separate A/D converter in each case.
 4. Arrangementaccording to claim 1, 2 or 3, characterised in that the scaling factors(s1, s2) are determined through a calibration procedure.
 5. Arrangementaccording to claim 1, 2, or 3 characterised in that the A/D converter orconverters are of the sampling type, and the weighting and scaling ofthe digital values is carried out for each sample value.
 6. Arrangementaccording to claim 1, 2, or 3, characterised in that there is arelationship, which is defined by a polynomial, between the weightingfactors (w1, w2) and the digital output value of one of the A/Dconverters.
 7. Arrangement according to claim 6, characterised in thatthere is a linear relationship.
 8. Arrangement according to claim 1, 2,or 3, characterised in that there is a relationship, which is defined bya piecewise linear function curve, between the weighting factors and theoutput of one of the A/D converters.
 9. Arrangement according to claim1, 2, or 3, characterised in that there is a relationship, which isdefined by a harmonic function, between the weighting factors and theoutput of one of the A/D converters.
 10. Arrangement according to claim1, 2, or 3, characterised in that the output voltage of each dioderectifier is converted into an AC voltage by using a chopper and, afteramplifying and digitising, is converted back again through synchronousdemodulation, the switching times of the chopper and synchronousdemodulator being synchronised with the measurement procedure so thatthe switching times respectively lie outside the measurement intervals.11. Arrangement according to claim 10, characterised in that the startof the measurement intervals is determined by a trigger signal which isderived from the measurement signal or is externally provided, and theduration of the measurement intervals is determined by an aperture timewhich is freely selectable or is derived from the measurement signal,and the chopper and synchronous demodulator are alternately in thenon-inverting and inverting states, respectively, for one period of themeasurement signal.
 12. Arrangement according to claim 1, 2, or 3,characterised in that the digital values are corrected with respect totheir offset and/or are linearised before weighting and scaling. 13.Arrangement according to claim 1, 2, or 3 characterised in that theoutput voltages of the diode rectifiers (B1, B2) are in each caseconverted into proportional digital values.